Method of using induction furnaces



Ma 13,1969 GALEY ET AL 3,443,806

METHOD OF USING INDUCTION FURNACES Filed Aug. 10, 1966 Sheet Of 2'Avwavrow Jew/v 6'41 5 v Guaeve-r 1/4: l/

hr s May 13, 1969 J GALEY ET AL I 3,443,806

METHOD OF USING INDUCTION FURNACES Filed Aug. 10, 1966 Sheet 2 of 2"/T(5TP) o 1 T I f 1 awn/me;

United States Patent 3,443,806 METHOD OF USING INDUCTION FURNACES JeanGaley, Ville dAvray, and Gilbert Valin, Noiseau, France, assignors toLAir Liquide, Paris, France Filed Aug. 10, 1966, Ser. No. 597,185 Int.Cl. C2lc 5/00 US. Cl. 266-34 5 Claims ABSTRACT OF THE DISCLOSURE Ametallurgical induction furnace is operated by imposing an alternatingelectromagnetic field on molten metal in a crucible, and blowing finebubbles of gas through a lower porous surface of the crucible. Amongother advantages, the trapping of solid impurities against the walls ofthe crucible is prevented.

The present invention relates to amethod of using metallurgicalinduction furnaces, more particularly those using a medium frequency andnot employing a vacuum.

It is characterised in that an agitating gas in the form of fine bubblesin blown in at the lower portion of the crucible of the furnace, thisgas supplementing and correcting the electromagnetic agitating effects.

Users of special alloys and steels are stressing more and more the needfor providing metals of high quality, and more particularly for beingable to rely on the uniformity of the properties of such metals.

Of these properties, purity, characterised by the absence of inclusionsand homogeneity are particularly important.

In order to obtain uniformity of such properties, use is frequently madeof operating under a vacuum or in a protected atmosphere.

The use of a vacuum requires a relatively expensive and complicatedapparatus and does not solve all the problems. It is known moreparticularly that treatment under a vacuum cannot itself ensure completedecantation of solid impurities nor the total elimination of occludedgases.

More particularly, in the case of an induction furnace, the flow of themetal caused by the action of the alternating magnetic field occursalong more or less complicated paths of travel, in which descendingcurrents are formed along the walls of the furnace. These currents,which are described more particularly in the work, Metallurgie du fer(Dunod, 1957, 2nd edition), pages 311-312 by L. Colombier, result in anirreversible trapping of the inclusions by the walls of the furnace, Itwill be appreciated that the action of the vacuum cannot by itselfmodify this phenomenon.

According to the present invention it has been found that it is possibleto combine advantageously the effects of the electromagnetic agitationwith those of the blowing in of an agitating gas in fine bubbles throughat least one porous refractory element inserted in the bottom of thecrucible of an induction furnace.

Tests have in fact shown the possibility of using the blowing in of gasto correct the unfavourable effects of electromagnetic agitation, whilstpromoting on the other hand certain advantageous effects of thisagitation, more particularly as regards the homogenisation of thealloying elements which are heavier than iron, such as tungsten, or onthe contrary which are lighter than iron, when iron is the metal to betreated.

It has been found more particularly that the suitably directed blowingin of gas in fine bubbles not only enables the descending currents ofliquid metal to be deflected so as to move them away from the walls ofthe furnace, but also makes it possible to carry towards the surface ofthe molten bath the solid particles which have adhered to the walls.

3,443,806 Patented May 13, 1969 Preferably, a cover is arranged on theupper portion of the furnace crucible, this cover being provided ifdesired with a pipe for the introduction of inert gas. The usefulness ofthis cover, often very considerable, is to create above the bath oninert atmosphere emanating from the agitating gas and with which theambient air does not mix.

In the accompanying drawings:

FIGURE 1 shows diagrammatically the flow of the metal in a conventionalinduction furnace.

FIGURE 2 shows diagrammatically this flow modified by the blowing in ofan agitating gas according to the invention.

FIGURE 3 ShOWs in section a portion of one form of embodiment of theinvention.

FIGURE 4 is a graph showing the consumption of agitating gas in variouscases.

In FIGURES 1 and 2 induction furnaces have been shown which are open tothe ambient air and whose crucible 1 is surrounded by a heating solenoid2 through which flows an alternating current at medium frequency (ofabout 500 to 2000 cycles per second).

The furnace in FIGURE 1 is a typical example of the known art whereasthe furnace shown in FIGURE 2 has had the improvement which forms thesubject of the present invention applied to it.

In the furnace shown in FIGURE 1 currents 3 flow in the liquid metal asfollows:

On the one hand about an equatorial plane of symmetry HH', situatedsubstantially halfway up the height of the bath; and on the other handabout a vertical axis of symmetry XX substantially coincident with thevertical axis of the crucible.

At the upper portion of the bath, these currents 3 flow downwardly alongthe walls of the crucible, with the result that there is an irreversibletrapping of the solid impurities of the metal, which are constantly keptin contact with the walls of the crucible, mainly in the lower portionof the said crucible.

In FIGURE 2 the induction furnace is provided with a porous refractoryplug 4 inserted in the lining of the crucible bottom. A conduit 5 isused for introducing a gas under pressure through the porous plug. Thisgas, entering into the metal at the bottom of the crucible, gives riseto a rapid ascending current 6 constituted by an emulsion of gas in themetal, in which the gas is dispersed in a multitude of fine bubbles eachof which is capable of fixing, by adsorption, one or more solidparticles and of entraimng even those which have adhered to the wallsbefore the blowing-in operation has commenced. It is to be noted thatthe current 6 is much more rapid than the currents 3 which are producedby the alternating magnetic field.

On the other hand, the blowing in of an inert gas in fine bubblesthrough the bottom of the furnace promotes the release of occluded gasessince each bubble of inert gas constitutes a micromedium in which theoccluded gas is initially at a zero partial pressure, so that theoccluded gases tend to issue from the metal and pass into the bubble,until the partial pressure is balanced. The total' surface of thebubbles is greater in proportion as their diameter is smaller, and thisfactor plays an important part in the decantation of the solidimpurities and the elimination of occluded gases.

Another favourable effect of blowing in gas is that it opposes theconcentration of the heavy additions, such as ferrotungsten, in thebottom of the furnace, and the concentration of the light additions inthe upper layers.

Blowing in may take place either whilst electromagpetic agitation istaking place or whilst the electromagnetic arrangement is inoperative.

Finally, a subsidary result of considerable interest is that the metalis protected not only during its preparation but until it is poured whenthe furnace is tipped, owing to the presence of a layer of inert gascovering the surface of the bath. This surface is thus protected fromcontact with the atmosphere. 7

The blowing in of inert gas through the bottom ensures that this gas,after having passed through the bath of molten metal, covers the entiresurface of the bath; in this respect, this method of blowing in isreliable and more advantageous than a method consisting in using lancesor nozzles directed towards the surface of the bath, which areconsequently capable of entraining air against the said surface by anejector effect.

Consequently the blowing in of inert gas through the crucible bottomachieves, in conjunction with electromagnetic agitation, an improvementin the efficiency of readily slaggable additions (titanium, aluminum,calcium etc.); in fact, in current induction furnace practice, an oftenvery considerable proportion of these additions is burned in contactwith air before there is a possibility of being incorporated in theliquid metal.

The invention also relates, as a novel apparatus, to an inductionfurnace particularly of a medium-frequency type the bottom of which isprovided with one or more porous plugs arranged in such a manner as topermit the blowing in of gas under the conditions defined hereinbefore.

The use of a porous plug permits of introducing the gas at the lowerportion of the metal bath, contrary to what occurs when using a lancewhich enters the metal bath, and a lance is also much more difiicult toprotect from the heat of the bath and the corrosive action thereof.

Furthermore, the improvement provided by the invention, which consistsin making the gas agitation and the electromagnetic agitation effectssupplement one another, can considerably widen the field of applicationof the electric furnace, more particularly by making it possible toagitate the metal in contact with the slag in this furnace, in themanner similar to that used in the Ugine- Perrin process.

The agitation may be applied more particularly to desulphurisation anddephosphorisation operations in the induction furnace. It may also beused so as to deoxidise steels and, in operations for the production ofspecial cast irons, so as to bring the graphite to a modular form. Inthese various operations, the inert gas can serve as a carrier for anactive gas, for example a reducing gas, or the vapours of a relativelyvolatile metal such as sodium or magnesium.

FIGURE 3 represents diagrammatically and by way of a nonlimitingexample, the upper portion of an induction furnace similar to that shownin FIGURE 2, in a vertical section taken through its axis.

The crucible 23 of the furnace and its induction coil 2 are arrangedbelow a floor 30. In the bottom of the crucible, not shown, there isarranged a refractory porous plug through which an agitating gas isblown in; bubbles of this gas are shown at 32, 34 for example, in themetal bath 36 to be treated. A cover closes the upper opening of thecrucible except for its pouring spout 38 if appropriate. This cover ismade of sheet metal 40 lined internally with refractory material 42. Itis supported on the crucible. A seal is provided if appropriate by arefractory mortar 44.

The cover prevents the oxidation of the metallic bath by the ambientair. It creates an inert atmosphere between it and the bath, emanatingfrom the agitating gas. It may be advantageous to provide a pipe 46through which an inert gas is injected so as to create the protec' tiveatmosphere more quickly and to maintain it more satisfactorily. Thecover may be designed so as to close the pouring spout also.

Experience has shown that the rate of flow of agitating gas is to bebetween 0.2 and 2 litres per minute per square centimetre of porous plugsurface in contact with the molten metal to be treated. If the flow rateis too low, there is inadequate agitation, and the provision of theporous plug and its gas supply is not worthwhile. If the flow rate istoo high, the effectiveness of the gas is much less.

In cases where the agitating gas is used also to degasify the metal, itsconsumption depends more particularly on the degree of degasificationwhich it is desired to achieve, and the output rate. This consumption isgenerally between 1 and 15 m. of gas per ton of metal, preferablybetween 2.3 and 10 m all gas volumes herein being given at standardtemperature and pressure.

FIGURE 4 relates to the dehydrogenation of steel by the injection ofargon. It shows how the quantity of gas consumed varies, this beingplotted as ordinates in m! per ton, as a function of the specific flowrate, which is plotted as albscissae and is counted in litres per minuteand per cm. of blowing in surface at the plug.

Each of the curves relates to a specific dehydrogenation of the treatedsteel:

The curve 48 relates to a reduction of from 5 to 3 cm. of hydrogen perg. of steel.

The curve 50 to a reduction of from 4 to 2 cm. per 100 g.

The curve 52 to a reduction of from 4 to 1.5 cm. per 100 g.

It will be seen from these curves that it is advantageous to use lowflow rates, but limits are set to this by the slowing down of theoperation.

What We claim is:

1. A method of treating molten metal in a metallurgical inductionfurnace, comprising imposing an alternating electromagnetic field onmolten metal in a crucible at a medium frequency of about 500 to about2,000 cycles per second, blowing -fine bubbles of inert gas through alower porous surface of the crucible in a quantity about 1 to about 15cubic meters at standard temperature and pressure per ton of moltenmetal, and maintaining the pressure at the surface of the molten metalat least atmospheric.

2. A method as claimed in claim 1, and confining to the regionimmediately above the surface of the molten metal the inert gas thatemerges from the molten metal.

3. A method as claimed in claim 2, and initially purging the said regionimmediately above the molten bath with said inert gas.

4. A method as claimed in claim 1, the flow rate of said gas being fromabout 0.2 to about 2 liters per minute and per square centimeter of saidporous surface that contacts the molten metal.

5. A method as claimed in claim 1, said inert gas being supplied in aquantity about 2.3 to about 10 cubic meters under standard temperatureand pressure per ton of molten metal.

CHARLES J. MYHRE, Primary Examiner.

3/1966 Fink].

